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Re: what is the FOS against overturning of structures....

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-----Original Message-----
From: Khosrownia, Ghassem SPK <GKhosrownia(--nospam--at)spk.usace.army.mil>
To: 'seaint(--nospam--at)seaint.org' <seaint(--nospam--at)seaint.org>
Date: Thursday, July 29, 1999 10:24 AM
Subject: what is the FOS against overturning of structures....


>What kind of FOS have you been using for the overall overturning stability
>of the whole structure? I am not finding anything in UBC or Reinforced
>Masonry Engineering Handbook.
>
None, directly.  However,  the FOS is implied in the foundation design
process.
For example, to determine maximum soil bearing pressure appropriate
combinations of load are applied to a shearwall/foundation assembly. The
resulting eccentricities will lead to soil bearing pressure maxima  which
are compared to possibles.
Specifically 0.85 dead and 1.0 "service" overturning compared to 1.33
allowable soil bearing, AND  0.9 dead and 1997 UBC overturning (i.e.;
"factored" seismic) compared to ultimate soil strength. Outside zone 4
combinations which include live load are more likely to control,  and so
forth for wind etc. That's the idea without getting tedious.
Please note that the practice of finding "service" soil bearing stress and
multiplying by 1.4 does not conform to UBC wording and is unconservative and
statically incorrect.
In short, if statics shows equilibrium the fos is at least one. If ultimate
soil pressure does not exceed ultimate soil strength fos is that established
by the geotech.
Pile foundations will show a similar pattern.